The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services from geostationary orbit. Market demands for such spacecraft have imposed increasingly stringent requirements on spacecraft payloads. For example, service providers desire spacecraft with payloads offering reconfigurable service to multiple coverage areas.
Satellite payloads generally include one or more antenna systems configured to project beams of a certain shape for transmitting and/or receiving radio waves. For example, an antenna system of a geostationary satellite may be configured to project a beam that is roughly the shape of a geographic region, such as the boarders of a country, or a small “spot” beam covering a single city or region. As a further example, the satellite payload may be constrained to avoid transmitting to and/or receiving from certain regions in accordance with government regulatory requirements, and/or to avoid interference to and/or from other users of the same frequency spectrum in those regions. In order to maximize the utility of a satellite, it is often desirable to operate as many beams as possible from a single satellite. In light of mass and envelope constraints, it is desirable to generate multiple beams from each antenna system.
As observed by Lyerly et al., in US Pat Pub 2004/0008148 (hereinafter “Lyerly”) a satellite reflector antenna may be configured to produce a shaped beam that corresponds to the shape of a particular market region by using an array of multiple feeds (a “feed array”) placed at the reflector focus region in order to produce the desired shape in the antenna far field pattern. For example, by varying the amplitude and phase excitation of each feed in the feed array, multiple desired beam shapes may be generated. A combiner network is used to distribute energy to each of the many feeds required to produce the shaped beam. The consequential result is an increase in weight and volume to the satellite antenna system as well as a risk of undesirable electrical coupling between feeds. Moreover, there is very limited capability to reconfigure the shaped beam, the parameters of which are largely fixed by initial design of the feed array.
A requirement for multiple feeds may sometimes be avoided by employing a shaped main reflector and/or sub-reflector or both. By “shaped”, as used herein and in the claims, is meant that a substantially paraboloid, ellipsoid, or hyperboloid reflector surface is additionally specially contoured so as to provide a desired beam shape. Because each feed element is associated with a single shaped beam, a need for a feed array and a combiner network may be substantially avoided. As a result, the weight, volume, cost and complexity of the antenna system are reduced. In the absence of the present teachings, however, an ability to independently steer and shape an individual beam, without affecting other beams produced by the antenna system, can be provided only with substantial penalties of mass, cost and reliability.
For example, in a known technique, one or more feeds may be provided that independently translate relative to the reflector or subreflector. The feeds must be connected to a transponder in order to receive and amplify radio signals. For the feed to move, it must be connected either with a beam waveguide, or flexible waveguide or cable. A beam waveguide requires multiple additional reflectors, is relatively heavy, and consumes a large amount of space. Flexible waveguide is prone to failure after repeated bending, and cable is lossy and cannot carry much power without overheating.
In view of the above, improved techniques for generating multiple independently-steered and shaped beams from a single antenna system are desirable.